Bevel gears blanks (i.e. ring gear and pinion), in particular ring gear blanks, are usually manufactured from forgings. The forgings include an amount of excess stock material so as to allow for machining in order to clean up the blank surface and achieve the correct blank dimensions.
Gear blank forgings are often dimensioned such that they can be used for all members of a part (i.e. gear) family. In this case, the stock allowance can be several millimeters. In the case of pinion members of part families, the pinion shank design is usually identical for all different members of the part family and only the pinion head (diameter, face angle etc.) varies among family members. The ring gears members of part families usually have an identical bore and back face geometry (e.g. bolt hole pattern, tapping, inner and outer diameter of the back face) while the front inside surface, front face and front outside surfaces usually vary among family members.
Pinion blanks are usually turned in one chucking (i.e. positioning and securing a workpiece for machining on a machine tool) between centers while ring gear blanks are usually turned in at least two chuckings. Smaller ring gears are mostly turned in two-spindle CNC lathes (i.e. two chuckings) while larger ring gears (e.g. diameter 400 mm and above) are often turned in two or even three separate machines (i.e. two or more chuckings). Subsequent to such machining, the gear blank is usually transferred to a bevel gear cutting machine where the teeth of the gear are formed.
As an alternative to multiple machines for blank machining and tooth cutting, US 2008/0152446 to Muller et al. teach a device for performing green machining of a bevel gear blank (e.g. drilling, turning, milling, etc.) at a pre-machining station followed by transfer of the gear blank to an adjacent machining station for cutting the bevel gear teeth.
Also, US 2008/0213055 to Ozdyk et al. teach a device for the soft machining of bevel gears comprising a lathe arrangement having a tool base and a tool housing both of which are translatable at least horizontally in the X and Y directions on the base of the lathe. The tool base comprises a tool carriage and a tool holder that is rotatable about an axis B2 with at least one tool (e.g. turning, drilling, etc.) being positioned on the tool holder for machining a gear blank. At least one of the tools on the tool holder is separately rotatable about another axis B4 arranged generally parallel to the tool holder axis B2. The tool housing includes a milling head comprising a gear cutting tool which is rotatable about an axis B3.
In an alternative embodiment presented by US 2008/0213055, a combined tool holder/tool housing arrangement on the lathe is discussed wherein the tools (e.g. turning, drilling, etc.) and the milling head are located on the rotatable tool holder with milling cutter axis B3 being positioned separate from but parallel to axes B2 and B4 and the tool carriage being made to pivot about yet another axis B5. However, with this alternative embodiment, machine stiffness and cutting precision is diminished due to the plurality of linear and rotational axes of the tool holder/tool housing involved in positioning the carriage, tools and milling cutter and rotating the tools and milling cutter. Furthermore, considerable time is dedicated to preparation of the gear blank and as such, actual gear cutting time on the machine is lessened.
Turning of a ring gear blank in two or more chuckings is time consuming, costly and in many cases leads to undesirable runout and inaccuracy of the bore, back face and front face. Inaccuracy and runout in the front face can lead to mesh interferences of the final bevel gear pair. Such interferences can cause a failure of a gear set. It would be advantageous, with respect to the manufacturing flexibility and economy as well as part quality, if ring gear and pinion blanks would only have to undergo a single chucking in a CNC lathe, preferably to machine those surfaces common to all members of the part (gear) family (no machining of the surfaces which vary within the part family). In the case of part families, the inventory of blanks could be minimized since subsequent front turning operations could be utilized to produce a blank of a desired size.